1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same. In particular, the present invention relates to a MOS (metal-oxide-semiconductor) semiconductor device, in which the reliability of thin dielectric films, such as gate oxide layers, is increased, and a method of manufacturing the same.
2. Related Art
Recently, as the performance and speed of LSIs (large scaled integrated circuits) are improved, the miniaturization of MOS transistors has advanced. Accordingly, there have also been rapid advances in decreasing the thickness of gate dielectric films of MOS transistors. As a result, there is a demand for a technique for uniformly forming an ultra-thin silicon dielectric film uniformly with a high reliability. It was recently reported that a device first suffering soft breakdown (SBD) and then reaching hard breakdown (HBD), as a dielectric breakdown mode, has a longer lifetime than a device first suffering hard breakdown (K. Okada et al., Tech Dig. Symp. on VLST Technology, p. 57 (2000); K. Okada et al., “A Concept of Gate Oxide Lifetime Limited by “B-mode” Stress Induced Leakage Currents in Direct Tunneling Regime”, Symposium on VLSI Technology Digest of Technical Paper, 1999, PP. 57–58). However, since whether a dielectric breakdown of a gate dielectric film is a soft breakdown or a hard breakdown is dependent on random chance, it has been very difficult to selectively cause only soft breakdown. A device suffering soft breakdown has a significantly longer lifetime (a period from the time a device suffers soft breakdown to the time it reaches hard breakdown) when it is used under a low voltage stress than when it is used under a high voltage stress. This is shown in FIG. 8, in which the horizontal axis represents stress voltage (−V) applied to the gate of a device, and the vertical axis represents lifetime (S). That is, THB shows the period (lifetime) from the time a device suffers soft breakdown to the time it reaches hard breakdown. As can be understood from FIG. 8, the gradient of the graph changes when the stress voltage is less than about 3.2 V. This means that when the stress voltage is under about 3.2 V, the lifetime is extended.
As described above, a dielectric breakdown of a gate dielectric film is believed that once a soft breakdown occurs then a hard breakdown occurs. However, in general, there has been a problem in that technically it is not possible to control the time from the point a soft breakdown occurs to the point a hard breakdown occurs.